When the weather is nice I like to spend time in the garden, working on my ham radio antennas or otherwise working in the yard listening to the ball game or just music. I could listen with my MP3 player and ear buds, but they don’t stay in place well as I am moving around, and it is not the safest thing to be wearing while using outdoor power machinery. I prefer to crank up my old style “boom box”. I can set the volume loud enough to cover a large area.

The only problem is that it uses 6 D cells. They are expensive to replace. For a while I used rechargeable batteries. That worked, but often times they would go dead right in the middle of the game if I was not diligent in keeping them charged. Many rechargeable D cells actually have AA cells inside so only have the short charge of the smaller cells.

I could have bought another set, but instead decided to use sealed lead acid batteries. I have a number of them, and usually have a couple charged at any given time. Since they are 7AH batteries and my radio only draws about 200ma full volume, they go a long time between charges. The cassette tape deck would draw more power, but I don’t use that.

Sealed batteries are often used in emergency lights. I was able to get a bunch for free when they were replaced in a building I work in. Code requires regular replacement, and they have a lot of life left when replacement time comes. You can also buy them new. The one I have is fairly heavy, but in my use I don’t have to move it around much.

The 6 D cells provide 9V volts when fully charged. The sealed battery is a 12V model and actually has a voltage of around 13.6V when fully charged. To avoid damage to the radio a regulator is required. I used a commercially available regulator kit. You can build your own with a LM7809 or LM7808 and a few capacitors. Just use the example circuit in the data sheet.

Voltage regulator in the battery compartment

I probably should have taken the radio apart and mounted the regulator inside so I could go back to D cells. I was lazy and just mounted it inside the battery compartment. I drilled a hole to install a power jack on the battery compartment cover. The regulator outputs are soldered to the battery connections as shown in the picture. A piece of foam holds it in place when the cover is on. You could also use double sided tape or any number of other mounting methods depending on the size of the circuit and shape of the battery compartment.

This radio has a plug to run off AC line voltage. If your radio didn’t have that ability adding a regulator like this would let you run it off a 12V wall wart if you wanted to use line power indoors.

On sunny days I can use a small solar panel to power the radio. I have an 8W model that puts out about 16V open circuit. It provides plenty of power when the sky is clear or has only thin clouds. It can’t quite generate enough juice on cloudy days or if the sun goes behind a large cumulus cloud. The next step is to build a small charging circuit. When I have it outdoors with the solar panel and the sun is bright enough it will run the radio and charge the battery. The battery can take over when a cloud blocks the sun. Of course if I turn the radio off the battery will charge faster.

I am happy with this modification to my boom box. The cost of the regulator is less than a single set of batteries!